A known image forming apparatus includes a multi-beam scanning unit. The multi-beam scanning unit includes a plurality of light sources for a single developing unit and is configured to form electrostatic latent images on a surface of a photosensitive body by a plurality of light beams emitted from the respective different light sources. In such an image forming apparatus, for example, one or more of an optical error, a mechanical error, and displacement of optics due to temperature increase may cause a change in an interval between electrostatic latent images formed on the surface of the developing unit by respective light beams emitted from the respective different light sources. As a result, the quality of an image to be formed may be deteriorated.
Such a known image forming apparatus has a function of adjusting an interval between electrostatic latent images to be formed by respective light beams emitted from the respective different light sources. More specifically, the image forming apparatus causes the multi-beam scanning unit to form solid marks using the plurality of light sources. Each solid mark is formed by a single one of the light sources and has a plurality of scan lines with no gap therebetween. The known image forming apparatus further includes sensors that output signals corresponding to respective positions of the marks formed on the surface of the photosensitive body. The image forming apparatus adjusts the interval between electrostatic latent images to be formed by respective light beams emitted from the respective different light sources based on the signals outputted from the sensors.